Light-emitting diodes (LEDs) are becoming more important to modern day life and it is envisaged that they will become one of the major applications in many forms of lighting such as automobile lights, traffic signals, general lighting, liquid crystal display (LCD) backlighting and display screens. Currently, LED devices are typically made from inorganic solid-state compound semiconductors, such as AlGaAs (red), AlGaInP (orange-yellow-green), and AlGaInN (green-blue). However, using a mixture of the available solid-state compound semiconductors, solid-state LEDs that emit white light cannot be produced. Moreover, it is difficult to produce “pure” colors by mixing solid-state LEDs of different frequencies. Therefore, currently the main method of color mixing to produce a required color, including white, is to use a combination of phosphorescent materials that are placed on top of the solid-state LED whereby the light from the LED (the “primary light”) is absorbed by the phosphorescent material and then re-emitted at a different frequency (the “secondary light”), i.e., the phosphorescent materials down convert the primary light to the secondary light. Moreover, the use of white LEDs produced by phosphor down-conversion leads to lower cost and simpler device fabrication than a combination of solid-state red-green-blue LEDs.
Current phosphorescent materials used in down converting applications absorb UV or mainly blue light and convert it to longer wavelengths, with most phosphors currently using trivalent rare-earth doped oxides or halophosphates. White emission can be obtained by blending phosphors that emit in the blue, green and red regions with that of a blue or UV emitting solid-state device. i.e., a blue light-emitting LED plus a green phosphor such as, SrGa2S4:Eu2+, and a red phosphor such as, SrSiEu2+ or a UV light-emitting LED plus a yellow phosphor such as, Sr2P2O7:Eu2+;Mu2+, and a blue-green phosphor. White LEDs can also be made by combining a blue LED with a yellow phosphor, however, color control and color rendering may be poor when using this methodology due to lack of tunability of the LEDs and the phosphor. Moreover, conventional LED phosphor technology uses down converting materials that have poor color rendering (i.e., color rendering index (CRI)<75).